1. Field of the Invention
The invention relates generally to lighting arrangements using light emitting diodes (LEDs) and more particularly to LED lighting arrangements for use in illuminating public spaces such as roads and bicycle paths.
2. Description of the Related Art
Reflector units for streetlights are designed to distribute the light as evenly as possible over the area to be illuminated with minimal disturbance of the vision by glare and blinding. The optical design should meet an optimal balance between mast height, light uniformity, illumination coverage and the angle of glare and blinding of the light.
Glare is defined as a difficulty seeing in the presence of very bright light. Glare is stronger when bright light shines frontally into the face of a viewer than when shining at an angle. For a street light, the frontal angle perceived by a viewer approaching the light is known as the threshold increment (Ti). This angle is generally specified by designers such that the light shines at an angle of not less than 20° with the horizontal axis. A form of cut-off using the lighting unit surround may be used to achieve this. Nevertheless, reflection and refraction of light passing through the transparent cover of the lamp can still give rise to glare and is also a cause of “light pollution”—light that is directed upwards. The extent to which glare reduction is actually achieved depends largely on the effectiveness of these measures.
A further important factor that determines glare is the perceived size of the source or light emitting area. The amount of light emitted from a source having a given light emitting area may be defined by its luminance and measured in candelas per unit area. In general, a given amount of light emitted uniformly from a large area leads to considerably lower glare than the same amount of light emitted from a smaller area.
Conventional light sources for street lighting have included incandescent, fluorescent and other discharge lamps. More recently, alternative low-energy designs have been developed using LED light sources which are of considerably higher luminance i.e. significantly more concentrated in terms of flux/mm2. This highly concentrated light intensity together with the monochromatic character of special LED light sources requires a novel approach to the optical design. An additional factor in the design is the physical size of the point source. As indicated above, these factors are especially significant in terms of glare, since a small, bright point source can cause glare or blinding at even large distances.
Known solid state light sources of this type generally use lens optics mounted onto the chip. Typically, LEDs have an encapsulation with integrated lens to create beams with a desired opening angle e.g. 10° or 70°. Narrow beams are advantageous in that they have increased intensity and can be directed to the farthest points of a road. Existing designs for street lighting have attempted to use clusters of LEDs with increased light concentration close to the threshold increment in order to provide uniform distribution of light on the road surface. Concentrating point sources using lenses or collimators does nothing to overcome the problems of increased glare due to excessive luminance since the light emitting area of the LEDs remains small and the luminance increases with the square of the lens opening angle.
A device is described in PCT patent publication WO2006/132533 in which solid state light sources are provided with a light processing unit provided to process the intensity and/or direction of the generated light in order to illuminate specific regions of a road surface. Additionally, the device is designed to emit light in a first wavelength region and in a second wavelength region. According to the disclosure, the lighting unit is designed to generate light having a dominant wavelength from the first wavelength region in such a way that the eye sensitivity of the human eye is dominated by rods. Light in the second wavelength region is used for improving colour perception. Although the use of specific wavelengths can improve vision at low light intensity, the problems of glare remain.
Thus, there is a particular need for a lighting arrangement that combines the advantages of low power solid state light sources with reduced glare while providing a uniform light distribution over the road surface.